Helicopters have rotors driving blades. Rotation of the blades leads to drag movements that are known to manufacturers. Drag movements give rise to vibration in the form of oscillations, degrading the stability of the helicopter and comfort in flight. It is thus important to guarantee that there is no instability of the helicopter, in particular no ground resonance and no air resonance, the principles of which are explained in the paragraphs below. Drag movement also induces longitudinal and lateral forces in the fuselage that generate pitch and roll movements, and also parasitic movements in the drive system.
On the ground, the helicopter may be subject to a phenomenon of particular mechanical coupling known as “ground resonance” in the field of aviation. Thus, at a given speed of rotation for the rotor, the fuselage presents a vibration mode and the blades present a drag vibration mode. The blade movements induce forces on the rotor, giving rise to movements in the structure which in turn have an effect on the movements of the blades. If the frequencies of the fuselage and rotor modes are close enough together and if these modes are insufficiently damped, then the drag movements of the blades and the movements of the structure will amplify each other mutually and run the risk of becoming dangerous.
In order to act on this phenomenon, in addition to optimizing the stiffness and the damping of the landing gear, use is made of a drag damper mounted between two successive blades or between one blade and the rotor in order to avoid such coupling, by modifying the frequency of the drag movement, and where appropriate by applying damping to the drag vibration mode.
In flight, the helicopter can be subject to a resonance phenomenon of the same type as that described above, but coming from coupling between the drag vibration mode of the blades and the pendulum mode of oscillation of the fuselage. This phenomenon, known as “air resonance” is generally aggravated by severe flight configurations, such as turning for example, since the large amplitude of the forced dynamic response of the dampers as imposed by the rotor driving the blades then reduces their damping characteristics on the natural response which is at a frequency that is lower than that of the forced response.
If a drag damper presents a high degree of damping, then the hub which connects the blades to the rotary drive shaft of the rotor will be subjected to very large forces throughout the time of flight, leading to high levels of fatigue and to a risk of mechanical parts rupturing. This can be acceptable for stages of flight that are relatively short. In contrast, in a normal flight configuration, the drag dampers ought not to damp vibration unless it is generating instability, for example during particular flight configurations such as turning, etc. This can serve to reduce heating and consequently to reduce the risks of premature wear.
The degree to which the drag dampers damp therefore needs to adapt to operating and flight configurations, if it is desired to limit the forces on the hub when there is no risk of resonance.
Drag dampers are known in which it is possible to control the throttling of oil flowing from one chamber to another and back again via special valves. By way of example, such a damper presents a force/displacement relationship that corresponds to damper force depending on the amplitude of oscillation, and suitable for reducing vibration of the helicopter.
Such dampers often present reliability problems, and they are also very expensive.